The present invention relates to a detector constructed from electrically conducting fabric and configured to present a varying electrical characteristic in response to a mechanical interaction.
A fabric touch sensor for providing positional information is disclosed in U.S. Pat. No. 4,659,873 of Gibson. The sensor of Gibson is fabricated using two layers of fabric having conducting threads, where said conducting layers are separated by a resistive layer to prevent unintentional contact. The Gibson device is primarily an overlay for a visual display unit whereby the position of finger contacts may be identified in response to the display of representational icons, such as buttons etc. An electrical potential is applied across at least one of the layers and a voltage detected at a position of contact allows a position on the touch screen to be detected.
A problem with this configuration is that it is only capable of detecting a single touch and cannot identify two or more separate touches.
In some circumstances, it is desirable to provide a flexible detector constructed from electrically conducting fabric in which it is possible to detect two or more contact locations.
A proposal for achieving this is disclosed by the present applicants in British Patent application No. 2 341 932 and co-pending Australian patent application No. 48770/99, European patent application No. 99307539, Japanese Patent application No. 11-272513, Korean patent application No. 99-40363 and U.S. patent application Ser. No. 09/298,172. In these co-pending patent applications, one of the conducting planes is divided into a plurality of smaller planes, the operation of which is then time multiplexed so as to facilitate the detection of a plurality of mechanical interactions, provided that said interactions occur in different multiplexed regions.
A lower planar sheet is provided with connections at each of its corners to provide a two-dimensional co-ordinate position within the sheet area. An upper sheet is then divided into a plurality of portions and a mechanical interaction results in conducting planes of at least one of these portions being made active.
In order to achieve space division multiplexing of the regions, the electrical signals are time multiplexed such that operations upon each region are provided during a respective time slot. Each individual region is provided with its unique electrical connector established within the structure of this sheet.
Each output line associated with a region is provided with a respective buffering amplifier and a complete scanning cycle involves the application of a voltage between input terminals whereafter an output is considered from each of the individual output terminals.
A problem with this approach to providing a multiplexed sheet is that the construction of such a sheet is relatively difficult and thereby leads to significant constructional costs; thereby limiting its area of application. In addition, this approach requires the use of a 5-wire system, as distinct from the preferred 4-wire system, resulting in non-uniform electric fields and a requirement for compensation to be provided. This introduces further problems in terms of calibration and also in terms of loss of resolution.
An electrical switch using fabric elements is disclosed in British patent application No 1,308,575 in which conductive rows are made by coating portions of the fabric with an electrically conductive metal, such as silver or chromium. Manual pressure applied at particular regions may be detected by the intersection of a particular row and column being brought into contact but the presence of a continuous metal layer significantly restricts the flexibility of the device as a whole. This is emphasised by the fact that the device is to be constructed upon a rigid board and as such many of the benefits from using a fabric material are effectively lost.